Kyros

Sephiroth was controling Jenova's body, and the clones(including Cloud) through Reunion to bring him the Black Materia. He was the one pulling the strings.
On the other hand, Ghestal and the Empire would have gone after the Espers and the Warring Triad even if Kefka wasn't there.
For most of FFVI, Ghestal was the one pulling the strings and Kefka was just a pawn.

That's the difference between them.
Just because FFVI's world had a destroy-the-world-and-become-a-god 'button' named Warring Triad, and Kefka 'pushed' it, doesn't make him a more threatening or competent villain than Sephiroth.



But I have to admit that Kefka IS an entertaining character.